arXiv subjects

J. R. L. Santos

Publications and source records attributed to J. R. L. Santos.

At least 19 recordsLinked to original sources

arXiv2022

Reply to "Comment on `Energy Conditions in $f(Q)$ gravity'"

Recently, Avik De and L.T. How claim that we have not followed an effective theory approach for $f(Q)$ gravity, missing relevant terms to compute the energy conditions [Phys. Rev. D 102, 024057 (2020)]. They also state that we did not present how the pressure and energy density follow a set of energy condition criteria. In this reply we show how the effective density and effective pressure found in the comment yield to the same Fridmann Equations presented in our article. Therefore, we claim that the comment only introduces an equivalent path to derive the Friedmann Equations and that the extra terms found by Avik De and L.T. How are consequence of their specific definition for their effective density and effective pressure. However, such terms do not change the equations for $ρ$ and $p$. Consequently, these extra terms are not going to change the energy constraints presented by us in Phys. Rev. D 102, 024057 (2020).

gr-qc↗

arXiv2022

LIV effects on the quantum stochastic motion in an acoustic FRW-geometry

It is well known in the literature that vacuum fluctuations can induce a random motion of particles which is sometimes called quantum Brownian motion or quantum stochastic motion. In this paper, we consider Lorentz Invariance Violation (LIV) in an acoustic spatially flat Friedman-Robertson-Walker (FRW) geometry. In particular, we are looking for the LIV effects in the stochastic motion of scalar and massive test particles. This motion is induced by a massless quantized scalar field on this geometry, which in turn is derived from an Abelian Higgs model with LIV. Deviations in the velocity dispersion of the particles proportional to the LIV parameter are found.

gr-qc↗

arXiv2021

The Extension Method for Bloch Branes

The nature of gravity and its strength compared with other forces are fundamental challenges faced by the actual science. A path to understanding the hierarchy problem of gravity consists in adding an extra dimension to Einstein-Hilbert's general relativity. Therefore, gravity would be the only force leaking through this region of spacetime also known as brane. Despite the success of the mathematical description for the hierarchy issue through simple thin branes, it is also possible to derive a consistent theory of gravity using thick branes. The present work unveils a new procedure to determine analytic hybrid thick braneworld models. The discussions were based on the so-called Bloch branes, which are constructed via a two-field model coupled with gravity in 5-dimensional spacetime. Both fields used to build the Bloch branes depend only on the extra dimension, and the hybrid branes present internal structure, characterizing them as thick. In this work, we were able to derive new analytic thick brane models by using the extension method. Some of these new branes present a Minkowski spacetime immersed inside of the fifth-dimension. We showed how the symmetry of the emergent branes can be controlled via specific parameters. We also analyzed the stability conditions for each braneworld family, showing possible stable states for gravitons.

hep-th↗

arXiv2021

Constraining $f(Q,T)$ gravity from energy conditions

We are living in a golden age for experimental cosmology. New experiments with high accuracy precision are been used to constrain proposals of several theories of gravity, as it has been never done before. However, important roles to constrain new theories of gravity in a theoretical perspective are the energy conditions. Throughout this work, we carefully constrained some free parameters of two different families of $f(Q,T)$ gravity using different energy conditions. This theory of gravity combines the gravitation effects through the non-metricity scalar function $Q$, and manifestations from the quantum era of the Universe in the classical theory (due to the presence of the trace of the energy-momentum tensor $T$). Our investigation unveils the viability of $f(Q,T)$ gravity to describe the accelerated expansion our Universe passes through. Besides, one of our models naturally provides a phantom regime for dark energy and satisfies the dominant energy condition. The results here derived strength the viability of $f(Q,T)$ as a promising complete theory of gravity, lighting a new path towards the description of the dark sector of the Universe.

gr-qc↗

arXiv2020

Fast-roll solutions from two scalar fields inflation

One common approach for cosmic inflation consists in couple Einstein's gravity with a scalar field, often referred to inflaton field. In order to derive analytic simple scenarios, we usually work in the {\it slow-roll} regime. In such an approximation one considers the scalar field potentials to be nearly flat. It is possible to directly generalize such an approach for hybrid inflationary models, where the inflaton sector is composed of two or more scalar fields. However, the Friedmann equations and the equations of motion for such hybrid models are often hard to be analytically solved. Another recent path to describe cosmic inflation is through the so-called {\it fast-roll} regime, where one considers exactly flat potentials. Our purpose in this work is to obtain solutions for a hybrid inflaton model in the fast-roll regime. Cosmological interpretations through the behavior of Hubble, slow-roll, and equation of state parameters are also presented.

gr-qc↗

arXiv2020

Diffusive process under Lifshitz scaling and pandemic scenarios

We here propose to model active and cumulative cases data from COVID-19 by a continuous effective model based on a modified diffusion equation under Lifshitz scaling with a dynamic diffusion coefficient. The proposed model is rich enough to capture different aspects of a complex virus diffusion as humanity has been recently facing. The model being continuous it is bound to be solved analytically and/or numerically. So, we investigate two possible models where the diffusion coefficient associated with possible types of contamination are captured by some specific profiles. The active cases curves here derived were able to successfully describe the pandemic behavior of Germany and Spain. Moreover, we also predict some scenarios for the evolution of COVID-19 in Brazil. Furthermore, we depicted the cumulative cases curves of COVID-19, reproducing the spreading of the pandemic between the cities of São Paulo and São José dos Campos, Brazil. The scenarios also unveil how the lockdown measures can flatten the contamination curves. We can find the best profile of the diffusion coefficient that better fit the real data of pandemic.

q-bio.PE↗

arXiv2020

Energy Conditions in $f(Q)$ gravity

A complete theory of gravity impels us to go beyond Einstein's General Relativity. One promising approach lies in a new class of teleparallel theory of gravity named $f(Q)$, where the nonmetricity $Q$ is responsible for the gravitational interaction. The important roles any of these alternative theories should obey are the energy condition constraints. Such constraints establish the compatibility of a given theory with the causal and geodesic structure of space-time. In this work, we present a complete test of energy conditions for $f(Q)$ gravity models. The energy conditions allowed us to fix our free parameters, restricting the families of $f(Q)$ models compatible with the accelerated expansion our Universe passes through. Our results straight the viability of $f(Q)$ theory, leading us close to the dawn of a complete theory for gravitation.

gr-qc↗

arXiv2020

Inflation in $f(R,T)$ Gravity

The article presents modeling of inflationary scenarios for the first time in the $f(R,T)$ theory of gravity. We assume the $f(R,T)$ functional from to be $R + ηT$, where $R$ denotes the Ricci scalar, $T$ the trace of the energy-momentum tensor and $η$ the model parameter (constant). We first investigated an inflationary scenario where the inflation is driven purely due to geometric effects outside of GR. We found the inflation observables to be independent of the number of e-foldings in this setup. The computed value of the spectral index is consistent with latest Planck 2018 dataset while the scalar to tensor ratio is a bit higher. We then proceeded to analyze the behavior of an inflation driven by $f(R,T)$ gravity coupled with a real scalar field. By taking the slow-roll approximation, we generated interesting scenarios where a Klein Gordon potential leads to observationally consistent inflation observables. Our results makes it clear-cut that in addition to the Ricci scalar and scalar fields, the trace of energy momentum tensor also play a major role in driving inflationary scenarios.

gr-qc↗

arXiv2019

Cosmology from a running vacuum model driven by a scalar field

The possibility that the vacuum energy density $ρ_Λ$ is, indeed, varying in time, has been investigated lately from the running vacuum models perspective. Motivated by such models, in the present work, we relate the decaying vacuum energy $ρ_Λ(t)$ with a scalar field $ϕ$. We derive the equations of motion from such a premise and implement the first-order formalism in order to obtain analytical solutions to the cosmological parameters. We show that those are in agreement with recent Planck observational data. We discuss the physical consequences of having $ρ_Λ(t)$ related to $ϕ$.

gr-qc↗

arXiv2019

Brazilian Community Report on Dark Matter

This white paper summarizes the activities of the Brazilian community concerning dark matter physics and highlights the importance of financial support to Brazilian groups that are deeply involved in experimental endeavours. The flagships of the Brazilian dark matter program are the Cherenkov Telescope Array, DARKSIDE, SBN and LHC experiments, but we emphasize that smaller experiments such as DAMIC and CONNIE constitute important probes to dark sectors as well and should receive special attention. Small experimental projects showing the potential to probe new regions of parameter space of dark matter models are encouraged. On the theoretical and phenomenological side, some groups are devoted to astrophysical aspects such as the dark matter density profile while others explore the signature of dark matter models at colliders, direct and indirect detection experiments. In summary, the Brazilian dark matter community that was born not long ago has grown tremendously in the past years and now plays an important role in the hunt for a dark matter particle.

hep-ph↗

arXiv2019

Cosmology from an exponential dependence on the trace of the energy-momentum tensor -- Numerical approach and cosmological tests

In this paper, we present the cosmological scenario obtained from $f(R,T)$ gravity by using an exponential dependence on the trace of the energy-momentum tensor. With a numerical approach applied to the equations of motion, we show several precise fits and the respective cosmological consequences. As a matter of completeness, we also analyzed cosmological scenarios where this new version of $f(R,T)$ is coupled with a real scalar field. In order to find analytical cosmological parameters, we used a slow-roll approximation for the evolution of the scalar field. This approximation allowed us to derived the Hubble and the deceleration parameters whose time evolutions describe the actual phase of accelerated expansion, and corroborate with our numerical investigations. Therefore, the analytical parameters unveil the viability of this proposal for $f(R,T)$ in the presence of an inflaton field.

gr-qc↗

arXiv2019

A general method for transforming non-physical configurations in BPS states

In this work, we apply the so-called BPS method in order to obtain topological defects for a complex scalar field Lagrangian introduced by Trullinger and Subbaswamy. The BPS approach led us to compute new analytical solutions for this model.In our investigation, we found analytical configurations which satisfy the BPS first-order differential equations but do not obey the equations of motion of the model. Such defects were named non-physical ones. In order to recover the physical meaning of these defects, we proposed a procedure which can transform them into BPS states of new scalar field models. The new models here founded were applied in the context of hybrid cosmological scenarios, where we derived cosmological parameters compatible with the observed Universe. Such a methodology opens a new window to connect different two scalar fields systems and can be implemented in several distinct applications such as Bloch Branes, Lorentz and Symmetry Breaking Scenarios, Q-Balls, Oscillons, Cosmological Contexts, and Condensed Matter Systems.

hep-th↗

arXiv2018

Cosmological scenarios from multiquintessence

In this work we derive and analyse cosmological scenarios coming from multi-component scalar field models. We consider a direct sum of a sine-Gordon with a Z2 model, and also a combination of those with a BNRT model. Moreover, we work with a modified version of the BNRT model, which breaks the Z2 x Z2 symmetry of the original BNRT potential, coupled with the sine-Gordon and with the standard Z2 models. We show that our approach can be straightforwardly elevated to $N$ fields. All the computations are made analytically and some parameters restriction is put forward in order to get in touch with complete and realistic cosmological scenarios.

hep-th↗

arXiv2018

Harmonic oscillators from displacement operators and thermodynamics

In this investigation, the displacement operator is revisited. We established a connection between the Hermitian version of this operator with the well-known Weyl ordering. Besides, we characterized the quantum properties of a simple displaced harmonic oscillator, as well as of a displaced anisotropic two-dimensional non-Hermitian harmonic oscillator. By constructing the partition functions for both harmonic oscillators, we were able to derive several thermodynamic quantities from their energy spectra. The features of these quantities were depicted and analyzed in details.

cond-mat.stat-mech↗

arXiv2018

Lorentz-violating dimension-five operator contribution to the black body radiation

We investigate the thermodynamics of a photon gas in an effective field theory model that describes Lorentz violations through dimension-five operators and Horava-Lifshitz theory. We explore the electrodynamics of the model which includes higher order derivatives in the Lagrangian that can modify the dispersion relation for the propagation of the photons. We shall focus on the deformed black body radiation spectrum and modified Stefan-Boltzmann law to address the allowed bounds on the Lorentz-violating parameter.

hep-th↗

arXiv2018

Soliton solutions in two-dimensional Lorentz-violating higher derivative scalar theory

This paper shows a new approach to obtain analytical topological defects of a 2D Myers-Pospelov Lagrangian for two scalar fields. Such a Lagrangian presents higher-order kinetic terms, which lead us to equations of motion which are non-trivial to be integrated. Here we describe three possible scenarios for the equations of motion, named by timelike, spacelike and lightlike respectively. We started our investigation with a kink-like travelling wave Ansatz for the free theory, which led us to constraints for the dispersion relations of each scenario. We also introduced a procedure to obtain analytical solutions for the general theory in the three mentioned scenarios. We exemplified the procedure and discussed the behavior of the defect solutions carefully. It is remarkable that the methodology presented in this study led to analytical models, despite the complexity of the equations of motion derived from the 2D Myers-Pospelov Lagrangian. The methodology here tailored can be applied to several Lagrangians with higher-order derivative terms.

hep-th↗

arXiv2018

New family of potentials with analytical twiston-like solutions

In this letter we present a new approach to find analytical twiston models. The effective two-field model was constructed by a non-trivial combination of two one field systems. In such an approach we successfully build analytical models which are satisfied by a combination of two defect-like solutions, where one is responsible to twist the molecular chain by $180^{\,0}$, while the other implies in a longitudinal movement. Such a longitudinal movement can be fitted to have the size of the distance between adjacent molecular groups. The procedure works nicely and can be used to describe the dynamics of several other molecular chains.

hep-th↗

arXiv2018

Building analytical three-field cosmological models

A difficult task to deal with is the analytical treatment of models composed by three real scalar fields, once their equations of motion are in general coupled and hard to be integrated. In order to overcome this problem we introduce a methodology to construct three-field models based on the so-called "extension method". The fundamental idea of the procedure is to combine three one-field systems in a non-trivial way, to construct an effective three scalar field model. An interesting scenario where the method can be implemented is within inflationary models, where the Einstein-Hilbert Lagrangian is coupled with the scalar field Lagrangian. We exemplify how a new model constructed from our method can lead to non-trivial behaviors for cosmological parameters.

gr-qc↗